The present invention relates to an in-line electron gun for a color cathode ray tube and, more particularly, to an electric field correction plate for correcting aberration such as astigmatism in a main lens of the electron gun, in which a central aperture is of a key-hole shape and side apertures are of a asymmetric circular shape so as to make up for an assembly error caused by the application of the prior art elliptical shape to the main lens, thereby improving the assembly characteristics and the focusing characteristics.
Recently, in order to improve resolution characteristics of an in-line electron gun for a color cathode ray tube, the prior art separate type main lens becomes changed to the common type rim lens, and an enlarged aperture becomes applied to the main lens.
In this electron gun having the enlarged aperture, the difference in an opening size between the horizontal and the vertical direction causes the main lens to become asymmetric and the focusing forces to become different between the horizontal and the vertical directions, thereby causing aberration and distorting electron beams. That is, there is a problem that astigmatism and coma aberration occur in the main lens and thus exert a bad influence on the focusing characteristics.
In recent, an arrangement for compensating for such aberration is widely used wherein electric field correction plates are applied to the large aperture type main lens.
In a common electric field correction plate as shown in FIG. 3, three apertures 11 are arranged in line and the shapes of the apertures 11 are elliptical, which control the intensity of lens between the horizontal direction and the vertical direction by the change of the elliptical shape, and thus compensate for the aberration in the main lens. For clearly correcting the aberration of the main lens and improving the performance of the electron gun, the electric field correction plates 9, 10 must be assembled in the electron gun with high precision.
However, it is very difficult to assemble such electric field correction plate with high precision since the apertures in the electric field correction plates are elliptical, thereby deteriorating the whole focusing characteristics of the electron gun according to a lowering of the assembly precision.
To alleviate such a problem in Japanese Patent Hei 6-75378, only the central aperture of the electric field correction plate is elliptical and opposite side apertures of the electric field correction plate are circular, thereby improving the assembly characteristics. However, it is very difficult to correct astigmatism of opposite side beams by such electric field correction plate. Otherwise, there is a problem that the main lens should become complicated in the structure as compared with the prior art main lens.
Meanwhile, the in-line electron gun for a color cathode ray tube disclosed in U.S. Pat. No. 4,583,024, issued on Apr. 1, 1986, comprises a main focus lens formed by two spaced electrode members, each having three separate inline apertures therein, a central aperture and two side apertures. The improvement comprises each of the apertures in each of the focus lens electrodes having a shape that distorts a portion of the focus lens thereat, to at least partially compensate for an astigmatic effect without any further additional part. The side apertures in both of the electrodes are nonsymmetrical about axes that pass through the respective side apertures and are perpendicular to the initial coplanar paths of the electron beams. In this case, astigmatism is partially compensated for, but little effect can be obtained.
U.S. Pat. No. 4,833,364, issued to Izumida, et al., on May 23, 1989 discloses an electron gun for color picture tubes, wherein a focus grid and an anode made of elongated plates serves as main lens electrodes, and three apertures of the focus grid and the anode constitute a main lens assembly. The three apertures of each of the elongated plates have a central aperture defined by two first curves arcuated outward, and two side apertures each of which is defined by a second curve as an inner half arcuated inward and a third curve as an outer half arcuated outward, the first and second curves being less arcuated than the third curve. Elliptical rings are bonded to the elongated plates such that the end portions of the rings are constituted by semicircular portions aligned with those of the side apertures, thereby preventing astigmatism.
However, in such constitution, the shape of the inner wall in the ring electrodes and the shape of the elongated plates should be changed to control the path of the electron beams, thereby causing the change of the tube size to be difficult. That is, the anode voltage and the intensity of the main lens should be changed for the larger size tubes, but it is very difficult to change the path of the electron beams only by the change of the shape of the elongated plates since the cylindrical inner wall in the ring electrodes serves as an static electric lens electrode.
The present invention has been made to overcome the above described problems of the prior arts, and accordingly it is an object of the present invention to provide an in-line electron gun for a color cathode ray tube, which improves the assembly characteristics and compensates for aberration such as astigmatism, thereby substantially improving the focusing characteristics over the whole phosphor screen.
To achieve the above object, the present invention provides an in-line electron gun for a color cathode ray tube, which includes an electron beam generating means for generating and directing three electron beams along paths toward a phosphor screen, and a main lens of a large aperture type for focusing the three electron beams radiated from said electron beam generating means, said in-line electron gun for a color cathode ray tube being characterized in that said main lens is formed between a focusing electrode and an accelerating electrode; and electric field correction plates are disposed in each of the focusing electrode and an accelerating electrode in the vertical direction to electron beam passing axes, and have three electron beam passing apertures aligned in line, respectively, said three apertures comprising at least one central electron beam passing aperture of a key-hole shape which is symmetrical with respect to a vertical line to a central electron beam passing axis and two side electron beam passing apertures each of which is defined by a plurality of circular arcs having different radii of curvature, connected to each other circular arc, and asymmetrical with respect to a vertical line to each side electron beam passing axis.
It is preferred that each of said two side electron beam passing apertures of the electric field correction plates has the circular arc toward the central electron beam passing aperture smaller in radius of curvature than the circular arc far from the central electron beam passing aperture, and said plurality of circular arcs of the side electron beam passing apertures are connected by even-numbered straight lines.
Also, it is preferred that each of said two side electron beam passing apertures of the electric field correction plates has a shape of a key-hole toward the central electron beam passing aperture and a shape of an arcuate curve far from the central electron beam passing aperture. That is, each of said two side electron beam passing apertures of the electric field correction plates has a shape of a key-hole at one side with respect to a vertical line to each side electron beam passing axis and a shape of an arcuate curve at other side with respect to a vertical line to each side electron beam passing axis.
Furthermore, each radius of the circular arcs constituting said three electron beam passing apertures of the electric field correction plate disposed in the focusing electrode is preferrably smaller than each radius of the circular arcs constituting said three electron beam passing apertures of the electric field correction plate disposed in the accelerating electrode, and each of said three electron beam passing apertures of the electric field correction plate disposed in the focusing electrode may have a shape of a key-hole.